PROJECT SUMMARY Alzheimer?s disease (AD) is the most prevalent cause of late-onset dementia, affecting millions of people worldwide. Currently, there are no therapies that can halt or reverse AD. Hence, there is an urgent need to identify new therapeutic options. AD pathology is characterized by A? plaques and neurofibrillary tangles, which are linked to synapse loss and neuron death. Microgliosis is an additional pathological feature in regions affected by plaques and tangles. Recently, a rare variant in a microglial receptor, TREM2, was found to be associated with ~3-fold increased risk for sporadic AD, further implicating microglia in AD. TREM2 is a receptor for phospholipids that binds apoptotic cells and lipoprotein particles. The arginine-47-histidine variant associated with a high risk for AD impairs binding to phospholipids. Moreover, TREM2 deficiency and haploinsufficiency in mouse models of AD impair microglial clustering around plaques, which facilitates plaque spreading and damage of surrounding neurons. While optimal TREM2 function seems to protect from AD, it remains unclear how TREM2 sustains microglial responses during the progression of AD. In SPECIFIC AIM 1, we demonstrate that a defect in TREM2 in both mice and humans impairs mTOR signaling along with energetic and anabolic metabolism, and triggers compensatory autophagy, which, however, is insufficient to preserve microglial survival. We further show that this metabolic defect can be corrected by a creatine analog, cyclocreatine, which restores microglial ATP levels in vitro and promotes microgliosis around plaques and prevents neurite dystrophy in vivo. We will test whether, by improving metabolic fitness, cyclocreatine enhances microglial functions in vitro and cognitive function in fast and slow mouse model of AD in vivo. Additional TREM2 variants associated with increased risk of AD are found in the stalk region of TREM2, which undergoes protease cleavage by ADAMs proteases, resulting in the release of soluble TREM2 (sTREM2) at the expense of membrane-bound TREM2. In AD patients, sTREM2 levels in the cerebrospinal fluid correlate with microglial activation in response to neurodegeneration. Remarkably, recent in vitro studies showed that addition of sTREM2 to myeloid cells enhanced viability and suppressed apoptosis. In addition, injection of sTREM2 into the hippocampus of the mouse brain augmented microglia numbers. However, the functions of sTREM2 remain elusive. In SPECIFIC AIM 2, we demonstrate that sTREM2 binds to neurons in a human TREM2 transgenic mouse when crossed to mice that develop A? plaques. Therefore, we propose to test the hypotheses that AD spurs the production of sTREM2, and that full-length TREM2 promotes microglia activation, whereas sTREM2 has a separate, beneficial effect on neurons in vivo. To do this, we will monitor AD pathology in two novel transgenic TREM2 mouse lines that express either human sTREM2 or a modified human TREM2 that is not cleavable by proteases. Overall, this proposal will provide proof of principle that sustaining microglial metabolic fitness and production of sTREM2 are promising novel strategies for therapeutic intervention in AD.